Fuel cell system and related method

ABSTRACT

A fuel cell system (S) is provided with a fuel cell ( 1 ) supplied with hydrogen gas and oxidizing gas, a gas supply line ( 5 ) supplying the hydrogen gas to the fuel cell, a gas exhaust line ( 31 ) passing exhaust hydrogen gas expelled from the fuel cell with the exhaust hydrogen gas being able to contain nitrogen, a gas recirculation line ( 7 ) recirculating the exhaust hydrogen gas to the fuel cell at an upstream thereof, a purge valve ( 8 ) adapted to discharge the exhaust hydrogen gas, containing the nitrogen, to an outside, and a controller ( 100 ) operative to close the purge valve when judgment is made that a flow rate of the exhaust hydrogen gas passing through the purge valve during an opened state of the purge valve is equal to or greater than a predetermined value.

TECHNICAL FIELD

The present invention relates to a fuel cell system and its relatedmethod and, more particularly, to a fuel cell system and its relatedmethod wherein nitrogen is efficiently discharged from a hydrogen gasrecirculation line of a fuel cell in which hydrogen expelled from thefuel cell is recirculated for reuse.

BACKGROUND ART

With a polymer electrolyte fuel cell using hydrogen as fuel, excessivehydrogen expelled from a fuel cell stack is supplied to an inlet of thefuel cell stack whereby hydrogen is supplied to the fuel cell stack at alarger supply rate than that of hydrogen to be consumed in the fuel cellstack for enabling stable electric power generation.

Japanese Patent Application Laid-Open Publication No. 2001-266922discloses a fuel cell system that includes an ejector whose secondaryflow guide pipe is introduced with exhaust fuel, expelled from a fuelexhaust port of a fuel cell stack, through a check valve, with exhaustfuel expelled from the fuel cell being recirculated through the ejector.By recirculating excessive hydrogen expelled from the fuel cell stack toan inlet of the fuel cell stack through the ejector, it attempts toachieve stable electric power generation while supplying hydrogen to thefuel cell stack at a larger supply rate than that of hydrogen to beconsumed in the fuel cell stack without deserting the excessivehydrogen.

DISCLOSURE OF INVENTION

However, upon studies conducted by the present inventor, in such a fuelcell system in which the hydrogen is recirculated as described above,when using air as oxidizing agent, it is thought that nitrogen containedin air diffuses from a cathode to an anode through a membrane of thefuel cell stack. This leads to an increase in a nitrogen concentrationin the hydrogen.

If the nitrogen concentration in the hydrogen increases in such a way, adrop takes place in a hydrogen partial pressure, resulting indegradation in an electric power generating efficiency. Additionally, itis considered that a recirculation rate of the ejector that recirculatesthe hydrogen decreases and a stable electric power generation cannot bemaintained.

To address such an issue, it is considered that a purge valve isdisposed to allow the recirculated hydrogen to be discharged from ahydrogen recirculation passage to an atmosphere and the purge valve isperiodically opened to allow nitrogen to be discharged from the hydrogenline. However, it is conceived that if the purge valve is opened, thehydrogen is caused to be discharged together with nitrogen and, hence,if the purge valve is continuously opened for a time interval more thannecessary, degradation occurs in an efficiency of the fuel cell system.

Therefore, the present invention has been completed with the abovedescribed studies conducted by the present inventor and has an object toprovide a fuel cell system and its related method in which nitrogendiffused in a hydrogen line is discharged while minimizing the amount ofhydrogen to be concurrently discharged with nitrogen for providing ahigh operating efficiency.

To achieve the above object, one aspect of the present invention is afuel cell system which comprises: a fuel cell supplied with hydrogen gasand oxidizing gas; a gas supply line supplying the hydrogen gas to thefuel cell; a gas exhaust line passing exhaust hydrogen gas expelled fromthe fuel cell, the exhaust hydrogen gas being able to contain nitrogen;a gas recirculation line recirculating the exhaust hydrogen gas to thefuel cell at an upstream thereof; a purge valve adapted to discharge theexhaust hydrogen gas, containing the nitrogen, to an outside; and acontroller operative to close the purge valve when judgment is made thata flow rate of the exhaust hydrogen gas passing through the purge valveduring an opened state of the purge valve is equal to or greater than apredetermined value.

Stated another way, another aspect of the present invention is a fuelcell system which comprises: a fuel cell supplied with hydrogen gas andoxidizing gas; gas supply means supplying the hydrogen gas to the fuelcell; gas discharging means for discharging exhaust hydrogen gasexpelled from the fuel cell to an outside, the exhaust hydrogen gasbeing able to contain nitrogen and the gas discharging means includingpurge means for purging the exhaust hydrogen gas, containing thenitrogen, to the outside; gas recirculation means for recirculating theexhaust hydrogen gas to the fuel cell at an upstream thereof; andcontrol means for controlling the purge means so as to close the purgemeans when judgment is made that a flow rate of the exhaust hydrogen gaspassing through the purge means during an opened state of the purgemeans is equal to or greater than a predetermined value.

In the meanwhile, according to the other aspect of the presentinvention, there is provided a method of controlling a fuel cell system,which has a fuel cell supplied with hydrogen gas and oxidizing gas, agas supply line supplying the hydrogen gas to the fuel cell, a gasexhaust line passing exhaust hydrogen line expelled from the fuel cellwith the exhaust hydrogen gas being able to contain nitrogen, a gasrecirculation line recirculating the exhaust hydrogen gas to the fuelcell at an upstream thereof and a purge valve adapted to discharge theexhaust hydrogen gas, containing the nitrogen, to an outside, the methodcomprising: closing the purge valve when judgment is made that a flowrate of the exhaust hydrogen gas passing through the purge valve duringan opened state of the purge valve is equal to or greater than apredetermined value.

Other and further features, advantages, and benefits of the presentinvention will become more apparent from the following description takenin conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a structure of a fuel cell system of afirst embodiment according to the present invention;

FIG. 2 is a flowchart illustrating control operation of the fuel cellsystem of the first embodiment;

FIG. 3 is a view illustrating the relationship between a nitrogenconcentration in a hydrogen line and a flow rate of hydrogen gasrecirculated through an ejector under a situation where a temperature ofhydrogen gas and a pressure of hydrogen gas in the hydrogen line arekept constant, with the abscissa representing the nitrogen concentrationC in the hydrogen line and the coordinate representing the flow rate FCof hydrogen gas recirculated through the ejector, in the firstembodiment;

FIG. 4 is a view illustrating the relationship between the nitrogenconcentration in the hydrogen line and the flow rate of hydrogen gasthat is purged per unit time under the situation where the temperatureof hydrogen gas and the pressure of hydrogen gas in the hydrogen lineare kept constant, with the abscissa representing the nitrogenconcentration C in the hydrogen line and the coordinate representing theflow rate FP of hydrogen gas that is purged per unit time, in the firstembodiment;

FIG. 5 is a view illustrating the relationship between the temperatureof hydrogen gas at an outlet of a fuel cell stack and the flow rate ofhydrogen gas that is purged under the situation where the nitrogenconcentration and the pressure of hydrogen gas in the hydrogen line arekept constant, with the abscissa representing the temperature T ofhydrogen gas at the outlet of the fuel cell stack and the coordinaterepresenting the flow rate FP of hydrogen gas that is purged per unittime, in the first embodiment;

FIG. 6 is a view illustrating the relationship between the pressure ofhydrogen gas at an inlet of the fuel cell stack and the flow rate ofhydrogen gas that is purged under the situation where the nitrogenconcentration in the hydrogen line and the hydrogen temperature such asat an inlet of a purge valve and the outlet of the fuel cell stack arekept constant, with the abscissa representing the pressure P of hydrogengas at the inlet of the fuel cell stack and the coordinate representingthe flow rate FP of hydrogen gas that is purged per unit time, in thefirst embodiment; and

FIG. 7 is a view illustrating a structure of a fuel cell system of asecond embodiment according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A fuel cell system and its related method of each embodiment accordingto the present invention are described hereunder in detail with suitablereference to the accompanying drawings.

First Embodiment

First, referring to FIGS. 1 to 6, a fuel cell system and its relatedmethod of a first embodiment of the present invention are described indetail.

FIG. 1 is a view illustrating a structure of the fuel cell system of thepresently filed embodiment.

As shown in FIG. 1, the fuel cell system S of the presently filedembodiment is comprised of a fuel cell stack 1 that uses hydrogen asfuel and air as oxidizing agent to generate electric power, a hydrogentank 2 that stores hydrogen gas, an ejector 6 through which the hydrogengas is recirculated, a pressure sensor 4 disposed in an upstream side ofthe fuel cell stack 1 to detect a pressure of the hydrogen gas at aninlet of the fuel cell stack 1, and a purge valve 8 disposed in adownstream side of the fuel cell stack 1 to allow nitrogen, contained inexhaust hydrogen gas expelled from the fuel cell stack 1, to bedischarged to the atmosphere together with the exhaust hydrogen.

More particularly, the fuel cell stack 1 is constructed of a pluralityof stacks of fuel cell structural bodies BE, with separators 1 d beinginterleaved between adjacent components, each of which includes a fuelelectrode (also referred to as a hydrogen electrode) 1 a and an opposingoxidizing electrode (also referred to as an air electrode) 1 b with asolid polymer membrane 1 c being interleaved. Also, while FIG. 1 shows,for the sake of convenience, only one fuel cell structural body BE thatis sandwiched between a pair of separators 1 d, 1 d, it is to be notedthat no limitation is intended to the number of pieces of stacks.

The hydrogen stored in the hydrogen tank 2 typically takes the form ofgas, which is supplied to the fuel cell stack 1 through a hydrogendelivery conduit 5 through which the hydrogen tank 2 and the fuel cellstack 1 is connected. Here, a supply rate of the hydrogen gas isregulated by a hydrogen pressure regulator valve 3 that is disposed inthe hydrogen delivery conduit 5 to enable to variably adjust arestriction rate thereof and then, such regulated hydrogen gas issupplied to the fuel cell stack 1.

The ejector 6 is disposed in the hydrogen delivery conduit 5 between thehydrogen pressure regulator valve 3 and the fuel cell stack 1. Excessivehydrogen expelled from the fuel cell stack 1 is returned to an intakeport 6 a of the ejector 6 through a hydrogen return conduit 7 extendingfrom an exhaust hydrogen conduit 30, disposed downstream of the fuelcell stack 1, at an upstream side of the purge valve 8, therebypermitting the hydrogen gas to be recirculated to the fuel cell stack 1through the ejector 6. This allows the fuel cell stack 1 to be operativeto maintain stable electric power generation while enabling a reactionefficiency to be increased.

The pressure sensor 4 is disposed in the hydrogen delivery conduit 5between the ejector 6 and the fuel cell stack 1 in connection with aninlet of the fuel cell stack 1 and detects a pressure of the hydrogengas to be supplied to the fuel cell stack 1.

The purge valve 8 remains closed during normal operation of the fuelcell stack 1 and serves as a valve that when a concentration of thenitrogen, diffused from the air electrode 1 b of the fuel cell stack 1into a hydrogen line, increases in the hydrogen line, is operative toexpel the nitrogen to the outside, together with the excessive hydrogen,that is, the exhaust hydrogen gas expelled from the fuel cell stack 1.Here, in theory, the hydrogen line includes a supply line starting fromthe hydrogen tank 2 and extending to the purge valve 8 through thehydrogen delivery conduit 5 located with component elements such as theejector 6 and the pressure sensor 4, the fuel cell stack 1 and theexhaust hydrogen conduit 30 and, further, includes a recirculation lineextending to the ejector 6 and connected to the hydrogen deliveryconduit 5 through the exhaust hydrogen conduit 30 and the hydrogenreturn conduit 7. However, an objective region to be estimated as aregion, in which the nitrogen diffused from the air electrode 1 b of thefuel cell stack 1 is present, is a line comprised of the ejector 6, thehydrogen delivery conduit 5 extending from the ejector 6 to the fuelcell stack 1, the fuel cell stack 1, the exhaust hydrogen conduit 30extending from the fuel cell stack 1 to the purge valve 8, the hydrogenreturn conduit 7 connected to the exhaust hydrogen conduit 30 andextending to the ejector 6, and the purge valve 8 and, in the presentlyfiled embodiment, such a line is specifically termed the “hydrogenline”.

Disposed at an upstream of the ejector 6 are a pressure sensor 20 thatdetects a pressure of the hydrogen gas in the hydrogen delivery conduit5 at the upstream of the ejector 6, and a temperature sensor 22 thatdetects a temperature of the hydrogen gas in the hydrogen deliveryconduit 5 at the upstream of the ejector 6.

For the purpose of detecting a temperature of the exhaust hydrogen gasat an inlet of the purge valve 8, a temperature sensor 21 is disposed atthe inlet of the purge valve 8. The temperature of the exhaust hydrogengas, at the inlet of the purge valve 8, to be detected by thetemperature sensor 21 is used for determining a flow rate threshold,which will be described later, for judging whether purging isterminated. Also, in alternative, the temperature sensor 21 may belocated in the hydrogen return conduit 7, as shown in FIG. 1, with nointention to be limited, and may be located in the exhaust hydrogenconduit 30.

In the meanwhile, with respect to the air line, air is supplied from acompressor 9 to the fuel cell stack 1 through an air delivery conduit 10and exhausted to the atmosphere through an exhaust air conduit 31 and avariable restriction valve 11, serving as an air pressure regulatordisposed on a downstream of the exhaust air conduit 31.

Further, disposed in a coolant passage 12 for cooling the fuel cellstack 1 is a radiator 13 for heat dissipation, with coolant beingcirculated by a coolant pump 14.

Furthermore, an ammeter 1A for detecting output current of the fuel cellstack 1 is mounted on the fuel cell stack 1.

Moreover, detected information resulting from various sensors such asthe ammeter 1A, the pressure sensors 4, 20 and the temperature sensors21, 22 are delivered to a controller 100 and, using such detectedinformation, the controller 100 controls the various component elementsof the above-described fuel cell stack 1 at needs.

Next, a basic sequence of a control operation of the fuel cell systemwith the structure set forth above is described mainly with reference toa flowchart of FIG. 2. Also, a control processing related to such acontrol operation is executed by the controller 100 at appropriatetimings after start-up of the fuel cell system.

As shown in FIG. 2, first in step S1, judgment is made to find whetherthe purge valve 8 is currently opened and if the judgment is made thatthe purge valve 8 is open, the processing is routed to step S2. Incontrast, if in step S1, the judgment is made that the purge valve 8remains closed, the processing is routed to step S4.

In the next step S2, judgment is made to find whether a passing flowrate of the exhaust hydrogen gas in the purge valve 8 is equal to orexceeds a predetermined value, and if the judgment is made that thetransient flow rate of the exhaust hydrogen gas in the purge valve 8 isequal to or exceeds the predetermined value, the processing is routed tostep S3 whereupon in step S3, wherein the purge valve 8 is closed toterminate the current processing. On the contrary, in step S2, if thejudgment is made that the transient flow rate of the exhaust hydrogengas in the purge valve 8 remains less than the predetermined value, thepurge valve 8 remains open.

In contrast, if the judgment is made in step S1 that the purge valve 8remains closed, judgment is made in step S4 to find whether now is thetime for opening the purge valve 8 whereupon if the judgment is madethat now is the time for opening, the processing is routed to step S5where the purge valve 8 is opened to terminate the current processing.Such judgment for the time for opening such a purge valve 8 is conductedusing a drop in a cell voltage detected by the fuel cell stack 1 with nolimitation being intended thereto and it may be possible to setappropriate timings in which no hindrance occurs in opening the purgevalve 8. Incidentally, in step S4, if the judgment is made that now isnot the time for opening the purge valve 8, the current processing isterminated with the purge valve 8 being closed.

By the way, here, a study is conducted of the relationship between aconcentration C of the nitrogen in the hydrogen line and the flow rateFC of the recirculated hydrogen gas in which the excessive hydrogenexpelled from the fuel cell stack 1 is returned to the hydrogen deliveryconduit 5 through the exhaust hydrogen conduit 30, the hydrogen returnconduit 7 and the ejector 6.

FIG. 3 is a view illustrating the relationship between the nitrogenconcentration in the hydrogen line and the flow rate of the recirculatedhydrogen gas formed of the exhaust hydrogen gas via the ejector under asituation where the temperature of the hydrogen gas and the pressure ofthe hydrogen gas in the hydrogen line remain constant and the abscissarepresents the concentration C of the nitrogen in the hydrogen linewhile the coordinate represents the flow rate FC of the recirculatedhydrogen gas via the ejector.

Assuming that no hydrogen is mixed in the hydrogen line, although it canbe evaluated that the recirculated gas recirculating through thehydrogen line via the ejector 6 is substantially equivalent to thehydrogen gas, if the concentration of the nitrogen rises in the hydrogenline, a more increased volume of the nitrogen is liable to be mixed insuch recirculated gas with a resultant drop in a hydrogen partialpressure in the recirculated gas. This causes, as shown in FIG. 3, theamount of the hydrogen in the recirculated gas, which is the flow rateFC of the recirculated hydrogen gas, in the hydrogen line to decreasewith an increase in the concentration C of the nitrogen in the hydrogenline. Therefore, there is a need for the purge valve 8 to be opened forachieving the purge to lower the concentration of the nitrogen in thehydrogen line for thereby ensuring the flow rate of the recirculatedhydrogen gas with a view to obtaining a desired amount of the hydrogen.Namely, the maximum value C_(max) of the concentration of the nitrogenallowable in the hydrogen line is specified in relation to the minimumvalue FC_(min) of the flow rate of the recirculated hydrogen gasrequired in the hydrogen line, and for the purpose of precluding thenitrogen concentration C allowable in the hydrogen line from exceedingsuch a maximum value C_(max), there is a need for discharging therecirculated gas from the hydrogen line by opening the purge valve 8and, in brief, for purging the nitrogen. That is, it can be said thatthe recirculated gas may be purged from the hydrogen line by keeping thenitrogen concentration in the hydrogen line at a value of C (≦C_(max)).

However, when opening the purge valve 8 to purge the recirculated gasfrom the hydrogen line, not only the nitrogen is purged but also desiredhydrogen gas tends to be discharged to the atmosphere. Therefore, apurge time interval needs to be appropriately set for opening the purgevalve 8 and purging the recirculated gas from the hydrogen gas, that is,it is required to appropriately close the purge valve 8 and, next, sucha purge time is studied.

FIG. 4 is a view illustrating the relationship between the nitrogenconcentration in the hydrogen line and the flow rate of the exhausthydrogen gas that is purged per unit time under a situation where boththe temperature of the hydrogen gas and the pressure of the hydrogen gasin the hydrogen line are kept constant, with the abscissa representingthe concentration C of the nitrogen in the hydrogen line while thecoordinate represents the flow rate FP of the exhaust hydrogen gas thatis purged per unit time.

As seen from FIG. 4, the flow rate FP of the exhaust hydrogen gas thatis purged increases with a decrease in the concentration C of thenitrogen in the hydrogen line. When considering a situation where,supposing that the current concentration of the nitrogen in the hydrogenline remains at the value of C₁ (>C_(max)), the nitrogen concentrationdecreases from the maximum value C_(max), which is allowable, to a valueof C₂ (≦C_(max)) with a further increased margin, the flow rate of theexhaust hydrogen gas that is purged varies from a point FP₁ to a pointFP₂. This means that the presence of a situation where the flow rate ofthe exhaust hydrogen gas that is purged increases from the point FP₁ byΔFP (=FP₂−FP₁) to reach the point FP₂ enables the judgment that theconcentration of the nitrogen in the hydrogen line is adequatelylowered.

Consequently, when turning back to step S2 in the flowchart of FIG. 2for description, in a case where the judgment is made that the transientflow rate of the exhaust hydrogen gas in the purge valve 8 is equal toor exceeds the predetermined value, that is, when describing withreference to FIG. 4, that is, if the flow rate of the exhaust hydrogengas that is purged is judged to be equal to or greater than the value ofFP₂, this value is employed as a threshold FP_(TH) and the purge valve 8is closed with a view to terminating purge operation whereby, in theory,the purge valve 8 can be closed at a reasonable timing.

This allows the nitrogen concentration in the hydrogen line to belowered, while enabling to prevent the hydrogen from being wastefullydischarged.

Further, the relationship between the temperature of the exhausthydrogen at the outlet of the fuel cell stack 1 and the flow rate of theexhaust hydrogen gas that is purged is studied.

FIG. 5 is a view illustrating the relationship between the temperatureof the exhaust hydrogen gas at the outlet of the fuel cell stack and theflow rate of the exhausted hydrogen gas that is purged under a situationwhere the concentration of the nitrogen and the pressure of the hydrogengas in the hydrogen line are kept constant, with the abscissarepresenting the temperature T of the exhaust hydrogen gas at the outletof the fuel cell stack while the coordinate represents the flow rate FPof the exhausted hydrogen gas that is purged per unit time.

With the fuel cell stack 1 being comprised of a polymer electrolyte fuelcell employing the electrolyte membrane lc composed of a solid polymerelectrolyte, at the exhaust hydrogen conduit 30, that is, at the outletof the fuel cell stack 1 and the inlet of the purge valve 8, the exhausthydrogen gas expelled from the fuel cell stack 1 assumes a statuswherein steam is saturated or nearly saturated. For this reason, as thetemperature at the inlet of the purge valve 8 detected by thetemperature sensor 21 increases, the pressure of saturated steamdevelops and the flow rate of the gas at the outlet of the fuel cellstack 1 increases, with a resultant drop in the hydrogen partialpressure. Also, it may be considered that the temperature at the inletof the purge vale 8 and the temperature at the outlet of the fuel cellstack 1 are substantially equal to one another. That is, as shown inFIG. 5, as the temperature T at the inlet of the purge valve 8increases, that is, as the temperature T at the outlet of the fuel cellstack 1 increases, the flow rate FP of the exhaust hydrogen gas that ispurged tends to decrease. Supposing that the flow rate of the exhausthydrogen gas that is purged is set at the point FP₂ at which, in step S2in the flowchart of FIG. 2, the purge valve 8 is to be closed, it isunderstood that the flow rate FP₂ of the exhaust hydrogen gas that ispurged is sufficient to be set to a lower value as the temperature T atthe inlet of the purge valve 8, that is, the temperature T at the outletof the fuel cell stack 1 increases.

Accordingly, the threshold FP₂ (FP_(TH)) of the exhaust hydrogen gasthat is purged when closing the purge valve 8 can be set to a lowervalue as the exhaust hydrogen temperatures at the inlet of the purgevalve 8 and the outlet of the fuel cell stack 1 increase.

This enables the purge valve 8 to be closed while equally decreasing thenitrogen concentration in the hydrogen line regardless of thetemperature of the exhaust hydrogen gas.

Further, the relationship between the pressure of the hydrogen gas atthe inlet of the fuel cell stack 1 and the flow rate of the exhausthydrogen gas that is purged is described.

FIG. 6 is a view illustrating the relationship between the pressure ofthe hydrogen gas at the inlet of the fuel cell stack and the flow rateof the exhaust hydrogen gas that is purged under a situation where theconcentration of the nitrogen and the temperature of the hydrogen gas atthe inlet of the purge valve or that of the exhaust hydrogen gas at theoutlet of the fuel cell stack remain constant, with the abscissarepresenting the pressure P of the hydrogen gas at the inlet of the fuelcell stack while the coordinate represents the flow rate FP of theexhaust hydrogen gas that is purged per unit time.

As seen from FIG. 6, the flow rate FP of the exhaust hydrogen gas thatis purged decreases with a decrease in the pressure P of the hydrogengas at the inlet of the fuel cell stack 1. Supposing that the flow rateof the exhaust hydrogen gas that is purged when closing the purge valvein step S2 in the flowchart of FIG. 2 is set to the point FP₂, it isunderstood that the flow rate FP₂ of the exhaust hydrogen gas that ispurged when the purge valve 8 is to be closed is sufficient to be set toa lower value as the pressure P of the hydrogen gas at the inlet of thefuel cell stack 1 decreases.

Accordingly, the threshold FP₂ (FP_(TH)) of the flow rate of the exhausthydrogen gas that is purged when closing the purge valve 8 can be set toa lower value as the pressure P of the hydrogen gas at the inlet of thefuel cell stack 1 decreases.

This enables the purge valve 8 to be closed while equally decreasing thenitrogen concentration in the hydrogen line regardless of the pressureof the hydrogen gas.

Next, description is made of how the flow rate of the exhaust hydrogengas that is purged, that is, the flow rate of the exhaust hydrogen gasto be purged is obtained.

In particular,(i) the flow rate of the hydrogen gas supplied to the fuelcell system, (ii) the hydrogen gas consumed in the fuel cell system, and(iii) the flow rate of the hydrogen gas that is consumed for the purposeother than purging, are obtained and, from a resulting differencetherebetween, the flow rate of the exhaust hydrogen gas to be purged isobtained.

First, description is made of how the flow rate of the hydrogen gas thatis supplied to the fuel cell stack 1 within the fuel cell system isobtained.

In general, under a status where the ejector 6 takes the form of achoked condition, the flow rate of the hydrogen gas passing through thehydrogen pressure regulator valve 3 and supplied to the fuel cell stack1 can be calculated from the pressure and temperature of the hydrogengas at the area upstream of the ejector 6. In contrast, under a statuswhere the ejector 6 takes the form of an non-choked condition, the flowrate of the hydrogen gas passing through the hydrogen pressure regulatorvalve 3 and supplied to the fuel cell stack 1 can be calculated from thepressure upstream of and the pressure downstream of the ejector 6 andadditionally calculated in consideration of the upstream temperature ofthe ejector 6.

In the presently filed embodiment, since the nozzle of the ejector 6takes the form of restriction to permit the flow rate of the hydrogengas to be determined and takes the form of the choked condition, intheory, the flow rate of the hydrogen gas supplied to the fuel cellstack 1 can be calculated using the upstream pressure (supply pressure)and the downstream pressure (discharge pressure) of the hydrogen gaswith respect to the ejector 6 detected by the pressure sensor 20 and thepressure sensor 4, respectively. Also, in a case where remarkablevariation takes place in the temperature of the hydrogen gas supplied tothe fuel cell stack 1, the flow rate of the hydrogen gas supplied to thefuel cell stack 1 can be calculated in a more accurate fashion bycorrecting the flow rate of the hydrogen gas using the upstreamtemperature of the hydrogen gas with respect of the ejector 6 detectedby the temperature sensor 22.

Further, description is made of how the flow rate of the hydrogen gas,among the hydrogen gas consumed in the fuel cell system, consumed forthe purpose other than purging is obtained.

In first place, it may be considered that the flow rate of the hydrogengas consumed for the purpose other than such purging substantiallycorresponds to the flow rate of the hydrogen gas consumed in the fuelcell stack 1. Here, the flow rate of the hydrogen gas consumed in thefuel cell stack 1 is proportionate to the output current of the fuel ellstack 1. Accordingly, upon detection of the output current of the fuelcell stack 1 with the ammeter 1A mounted on the fuel cell stack 1, theflow rate of the hydrogen gas consumed in the fuel cell stack 1, namely,the flow rate of consumed the hydrogen gas for the purpose other thanpurging can be calculated.

Consequently, the flow rate of the exhaust hydrogen gas to be purged canbe obtained by subtracting the flow rate of the hydrogen gas, consumedfor the purpose other than purging, from the flow rate of the hydrogengas supplied to the fuel cell system both of which are obtained in suchways described above and, from the resulting flow rate of the exhausthydrogen gas for such purging, the flow rate of the exhaust hydrogen gasto be purged in step S2 in the flowchart of FIG. 2 can be set.

As set forth above, with the structure of the presently filedembodiment, it becomes possible to suppress the hydrogen from beingwastefully discharged together with the nitrogen due to the excessivepurging while precluding the electric power generation from beingunstable due to the insufficient discharge of the nitrogen. This enablesthe stabled operation to be accomplished, thereby realizing the fuelcell system with a high efficiency.

Further, the temperature sensor 21 is provided for detecting thetemperature of the gas passing through the purge valve 8, and thethreshold value for enabling the closure control of the purge valve 8 isset such that the higher the gas temperature, the less will be thethreshold value. This allows the amount of the hydrogen to be exhaustedto be appropriately suppressed regardless of the gas temperature, with aresultant capability of realizing the fuel cell system with a highefficiency.

Furthermore, the pressure sensor 4 is provided for detecting thepressure of the hydrogen gas in the hydrogen supply passage, and thethreshold value for enabling the closure control of the purge valve 8 isset such that the lower the gas pressure, the less will be the thresholdvalue. This allows the amount of the hydrogen to be exhausted to beappropriately suppressed regardless of the gas pressure, with aresultant capability of realizing the fuel cell system with a highefficiency.

Moreover, the flow rate of the exhaust hydrogen gas passing through thepurge valve 8 is obtained based on a difference between the amount ofthe hydrogen gas supplied to the fuel cell stack and the amount of thehydrogen gas consumed for the purpose other than that exhausted from thepurge valve 8. By so doing, no sensor is newly required for directlymeasuring the transient flow rate of the purge valve 8 and, using thevarious sensors such as the pressure sensors 4, 20 and the temperaturesensor 22 which are even used in a normal control mode, the flow rate ofthe gas passing through the purge valve 8 can be accurately obtained,thereby suppressing cost increase.

In addition, the flow rate of the hydrogen gas supplied to the fuel cellstack 1 is calculated based on the pressure of the hydrogen gas suppliedto the ejector 6, or such a supply pressure and a discharge pressurefrom the ejector 6. By so doing, the system that has no a flow ratesensor for detecting the flow rate of the hydrogen gas can be used, andtherefore cost increase can be suppressed.

Also, the flow rate of the hydrogen gas supplied to the fuel cell stack1 is calculated based on the pressure of the hydrogen gas supplied tothe ejector 6, or such a supply pressure and a discharge pressure andfurther the temperature of the hydrogen gas at the upstream of theejector 6. By so doing, the flow rate of the hydrogen gas can beaccurately calculated without the use of the flow sensor that detectsthe flow rate of the hydrogen gas.

Also, depending on the output current of the fuel cell stack 1 detectedby the ammeter, the amount of the consumed hydrogen is calculated. Thisenables the amount of the consumed hydrogen to be accurately calculatedthrough the use of the ammeter that would be used in normal control,thereby suppressing cost increase.

Incedentally, while the presently filed embodiment has been described inconnection with a case where the ejector 6 for the recirculatinghydrogen gas is used, the presently filed embodiment is not limitedthereto and the hydrogen gas may be recirculated using a pump or blowerin which even when using the pump or blower, since the hydrogen partialpressure drops as the nitrogen concentration increases like in the casewhere the ejector 6 is used and, therefore, a phenomenon occurs whereinthe amount of the hydrogen gas supplied to the fuel cell stack 1 is inshortage and thus, it is possible to control the nitrogen concentrationwith a similar structure. However, there is a probability in that theflow rate sensor is separately disposed for detecting the flow rate ofthe hydrogen gas supplied to the fuel cell stack 1.

Further, although the position at which the pressure of the hydrogen gasto be detected for obtaining the threshold value of the flow rate of theexhaust hydrogen gas for closing the purge valve 8 has been describedabove in connection with the inlet the inlet of the fuel cell stack 1,it is needless to say that it is not objectionable for the pressure ofthe hydrogen gas to be detected at the outlet of the fuel cell stack 1.Also, in a case where a remarkable hydrogen pressure loss occurs in thefuel cell stack 1, the use of the pressure at the outlet side enables anaccuracy to be increased.

Furthermore, while the hydrogen gas stored in the hydrogen tank 2 hasbeen used as the fuel gas, of course, the presently filed embodiment isnot limited thereto and another structure may be alternatively employedwherein fuel containing the hydrogen is reformed with a reformer atneeds to obtain the hydrogen gas.

Also, although the presently filed embodiment has been described withreference to the polymer electrolyte fuel cell employing the solidpolymer electrolyte as the electrolyte membrane 1 c of the fuel cellstack 1, of course, another structure may be utilized provided that atthe outlet of the fuel cell stack 1 or the inlet of the purge valve 8,the exhaust hydrogen gas expelled from the fuel cell stack 1 remains ina condition saturated with steam or in a condition nearly saturated withsteam.

Second Embodiment

Next, a fuel cell system and its related method of the present inventionof a second embodiment are described in detail mainly with reference toFIG. 7.

FIG. 7 is a view illustrating a structure of the fuel cell system of thepresently filed embodiment.

As shown in FIG. 7, the fuel cell system of the presently filedembodiment differs from that of the first embodiment mainly in that apressure sensor 24 is disposed upstream of the hydrogen pressureregulator valve 3 for detecting the pressure of the hydrogen gas toallow the flow rate of the supplied hydrogen gas to be calculated basedon the pressures at the upstream and the downstream of the hydrogenpressure regulator valve 3 detected by the pressure sensor 24 and thepressure sensor 20 respectively and, therefore, aiming at such adifferential point, like component parts bear the same referencenumerals while description is suitably given in a simplified form oromitted.

In particular, the opening degree of the hydrogen pressure regulatorvalve 3 is detected by an opening sensor 3 a mounted on the hydrogenpressure regulator valve 3, with resulting opening information beingdelivered to the controller 100 that controls closure of the hydrogenpressure regulator valve 3. And, the controller 100 obtains the flowrate of the hydrogen gas, on the same point of view as that of the firstembodiment wherein the flow rate of the hydrogen gas supplied to thefuel cell stack 1 is obtained, using the upstream pressure and thedownstream pressure of the ejector 6 in the first embodiment. That is,the flow rate of the hydrogen gas to be supplied can be calculated basedon the opening degree of the hydrogen pressure regulator valve 3 and theupstream pressure of the hydrogen gas and the downstream pressure of thehydrogen gas related to the hydrogen pressure regulator valve 3.

More particularly, in a case where the opening degree of the hydrogenpressure regulator valve 3 remains small in a choked condition, the flowrate of the hydrogen gas to be supplied is calculated based on only thepressure of the hydrogen gas at a far upstream than the hydrogenpressure regulator valve 3 detected by the pressure sensor 24. On thecontrary, in a case where the hydrogen pressure regulator valve 3remains in a non-choked condition, the flow rate of the hydrogen gas tobe supplied is calculated based on the pressure of the hydrogen gas atthe upstream of the hydrogen pressure regulator valve 3 detected by thepressure sensor 24, and the pressure of the hydrogen gas at thedownstream of the hydrogen pressure regulator valve 3 detected with thepressure sensor 20. Also, in the presently filed embodiment, thetemperature sensor 22 used in the first embodiment is not essential andmay be omitted.

Further, in the presently filed embodiment, as the temperature of thehydrogen gas to be supplied to the fuel cell stack 1, the temperature ofcoolant detected with a temperature sensor 23 disposed in the coolantflow passage 12 of the fuel cell stack 1 in place of the temperaturesensor 21 used in the first embodiment. That is to say, it is possibleto predict the temperature of the hydrogen gas based on the temperatureof coolant because heat exchange occurs between the hydrogen gas andcoolant within the fuel cell stack 1. Also, since the temperature ofcoolant can be more accurately detected than the temperature of thehydrogen gas, in a case where a remarkable change takes place in load ofthe fuel cell system and the temperature varies at a high speed, it isadvantageous in that the temperature of coolant is more accuratelydetected than the temperature of the hydrogen gas.

As set forth above, with the presently filed embodiment, the flow rateof the hydrogen gas to be supplied to the fuel cell stack 1 iscalculated based on the opening degree of the hydrogen pressure valve 3,disposed in the hydrogen delivery conduit 5 through which the hydrogengas is supplied into the fuel cell system, and the upstream pressure andthe downstream pressure of the hydrogen pressure regulator valve 3. Thisallows discharge of the exhaust hydrogen to be suppressed, with aresultant capability of realizing a fuel cell system with a highefficiency.

Moreover, while even the presently filed embodiment has been describedin conjunction with a case where the ejector 6 is used for recirculatingthe exhaust hydrogen gas, the present invention is not limited theretoand the exhaust hydrogen gas may be recirculated using a pump or ablower, similarly permitting the flow rate of the supplied hydrogen gasbased on the opening degree of the hydrogen pressure regulator valve 3and the upstream and downstream pressures of the hydrogen gas to enablean appropriate time for closing the purge valve 8 to be obtained.

Also, although the temperature sensor 22 used in the first embodimenthas been omitted, in a case where a change takes place in thetemperature of the hydrogen gas supplied to the hydrogen pressureregulator valve 3, by separately disposing a temperature sensor fordetecting the temperature of the hydrogen gas at the upstream of thehydrogen pressure regulator valve 3 and correcting the flow rate of thesupplied hydrogen gas based on the temperature of the hydrogen gasdetected with such a temperature sensor, of course, it is possible tocalculate the hydrogen supply flow rate at a further accurate fashion.

Third Embodiment

Next, a fuel cell system and its related method of a third embodiment ofthe present invention are described in detail;

The fuel cell system of the presently filed embodiment differs fromthose of the first and second embodiments mainly in that the flow rateof the hydrogen gas, consumed for the purpose other than purging, whichrepresents the flow rate of the hydrogen gas to be substantiallysupplied to the fuel cell system is calculated in a more strict fashionthan that achieved in the first and second embodiments and as astructure of the presently filed embodiment, any of the structures ofthe first and second embodiments can be theoretically employed.Therefore, aiming at such a differential point, like component partsbear the same reference numerals to describe the presently filedembodiment while suitably simplifying description or omitting the same.

In particular, under a situation where the fuel cell system is installedin a vehicle, since a remarkable load change takes place with a rapidspeed in variation, there is a probability wherein the pressure of thehydrogen gas to be supplied to the fuel cell stack 1 needs to be varied.

For example, in a case where the pressure of the hydrogen gas to besupplied to the fuel cell stack 1 is raised, the hydrogen gas issupplied to the fuel cell stack 1 at a larger flow rate than that of thehydrogen gas needed for generating electric power in the fuel cell stack1 and, conversely, in a case where the pressure of the hydrogen gas tobe supplied to the fuel cell stack 1 is lowered, the hydrogen gas issupplied to the fuel cell stack 1 at a further less flow rate. That is,this means that in a case where no consideration is taken for the flowrate of the hydrogen gas needed for additionally increasing ordecreasing the flow rate of the hydrogen gas to be supplied to thehydrogen line in order to increase or decrease the pressure of thehydrogen gas, a probability occurs where no accurate calculation of theflow rate of the exhaust hydrogen gas to be purged can be accomplishedduring a so-called transition period in which the pressure of thehydrogen gas changes.

Here, the flow rate per se of the hydrogen gas needed to be supplied tothe hydrogen line for increasing or decreasing the pressure of thehydrogen gas in the hydrogen line is proportionate to the amount of thechange in such a pressure with its proportionate coefficient beingdetermined in terms of a volume of the hydrogen line. As a consequence,in consideration of even a value of c×ΔP resulting from the amount ΔP ofthe change in such a pressure multiplied by the coefficient c determinedbased on the volume of the hydrogen line, the flow rate of the hydrogengas to be supplied to the fuel cell stack 1 within the fuel cell systemneeds to be obtained. In particular, the flow rate of the exhausthydrogen gas to be purged is to be obtained using the flow rate of thehydrogen gas obtained in consideration of the change of the pressure inthis manner, that is, the flow rate of the hydrogen gas obtained byadding the value of c×ΔP resulting from the amount ΔP of the change inthe pressure multiplied by the coefficient c determined based on thevolume of the hydrogen line to the flow rate of the hydrogen gas that isto be consumed due to the necessity for the electric power generation.

Accordingly, the flow rate of the exhaust hydrogen gas to be purged canbe calculated based on the amount of the change in the pressure of thehydrogen gas resulting from the amount of the change in the pressure ofthe hydrogen gas detected with the pressure sensor 4 shown in FIG. 1 orFIG. 7. This enables the calculation of the flow rate of the exhausthydrogen to be purged even during a time period covering the transitionperiod in which the pressure of the hydrogen gas changes.

As set forth above, with the presently filed embodiment, even in a casewhere the pressure of the hydrogen gas changes, it becomes possible toaccurately calculate the flow rate of the exhaust hydrogen gas to beconsumed for the purpose other than purging. This enables the flow rateof the exhaust hydrogen gas to be consumed for the purpose other thanpurging to be more accurately calculated and, thus, it becomes possibleto more accurately control the nitrogen concentration in the hydrogenline during a phase in which the purge valve 8 is closed.

According to the present invention set forth above, since it is soarranged that the purge valve is closed when the transient flow rate ofthe gas passing through the purge valve is equal to or exceeds thepredetermined value when the purge valve is opened, the nitrogencontained in the hydrogen gas can be adequately discharged from thehydrogen gas supply line while enabling to suppress the amount ofhydrogen from being exhausted. This enables an efficiency of a fuel cellsystem to be increased.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the teachings. The scope of the invention is defined withreference to the following claims.

INDUSTRIAL APPLICABILITY

According to the present invention set forth above, in a case where thetransient flow rate of gas passing through the purge valve is equal toor exceeds the predetermined value when the purge valve is opened, thepurge valve is closed and hence, nitrogen contained in hydrogen gas canbe adequately, enabling to increase an efficiency of a fuel cell system.Accordingly, such a fuel cell system can be applied to various fuel cellsystems in which undesired substances, such as nitrogen, other thanhydrogen is to be purged and can be applied to various devices such as afuel cell powered automobile and electric power generators forindustrial use or domestic use, with application being highly expectedin a widened range.

1. A fuel cell system comprising: a fuel cell supplied with hydrogen gasand oxidizing gas; a gas supply line supplying the hydrogen gas to thefuel cell; a gas exhaust line passing exhaust hydrogen gas expelled fromthe fuel cell, the exhaust hydrogen gas being able to contain nitrogenin addition to hydrogen; a gas recirculation line recirculating theexhaust hydrogen gas to the fuel cell at an upstream thereof; a purgevalve adapted to discharge the exhaust hydrogen gas, containingnitrogen, to outside; and a controller configured to calculate a flowrate of the exhaust hydrogen gas passing through the purge valve andclose the purge valve when the controller judges that the flow rate ofthe exhaust hydrogen gas calculated thereby is equal to or greater thana predetermined value.
 2. The fuel cell system according to claim 1,wherein the fuel cell includes a fuel electrode and an oxidizingelectrode with an electrolyte membrane being interleaved therebetween.3. The fuel cell system according to claim 2, wherein electrolyte of theelectrolyte membrane is comprised of a solid polymer electrolyte.
 4. Thefuel cell system according to claim 1, wherein the predetermined valueis set to be a value that reflects a concentration of nitrogen, in thegas supply line, returned to the upstream of the fuel cell through thegas recirculation line and contained in the exhaust hydrogen gas.
 5. Thefuel cell system according to claim 1, further comprising a temperaturesensor detecting a temperature of the exhaust hydrogen gas passingthrough the purge valve, wherein the predetermined value for use inclosing the purge valve is set to decrease as the temperature of theexhaust hydrogen gas detected by the temperature sensor increases. 6.The fuel cell system according to claim 1, further comprising a pressuresensor detecting a pressure of the hydrogen gas in the gas supply line,wherein the predetermined value for use in closing the purge valve isset to decrease as the pressure of the hydrogen gas detected by thepressure sensor increases.
 7. The fuel cell system according to claim 1,wherein the flow rate of the exhaust hydrogen gas passing through thepurge valve is obtained based on a difference between a supply rate ofthe hydrogen gas, supplied to the fuel cell, and a consumption rate ofthe hydrogen gas, among the hydrogen gas consumed in the fuel cell,except for the exhaust hydrogen gas discharged from the purge valve. 8.The fuel cell system according to claim 7, wherein the gas recirculationline includes an ejector, and the supply rate of the hydrogen gassupplied to the fuel cell is obtained based on a supply pressure ofhydrogen gas supplied to the ejector or the supply pressure of hydrogengas supplied to the ejector and a discharge pressure of the hydrogen gasdischarged from the ejector.
 9. The fuel cell system according to claim8, wherein the supply rate of the hydrogen gas supplied to the fuel cellis obtained further based on a temperature of the hydrogen gas at anupstream of the ejector.
 10. The fuel cell system according to claim 7,wherein the gas recirculation line includes a hydrogen pressureregulator valve regulating a pressure of the hydrogen gas supplied tothe fuel cell, and the supply rate of the hydrogen gas supplied to thefuel cell is obtained based on an opening degree of the hydrogenpressure regulator valve, and a pressure of the hydrogen gas at anupstream of the hydrogen pressure regulator valve or the pressure of thehydrogen gas at the upstream of the hydrogen pressure regulator valveand a pressure of the hydrogen gas at a downstream of the hydrogenpressure regulator valve.
 11. The fuel cell system according to claim10, wherein the supply rate of the hydrogen gas supplied to the fuelcell is obtained further based on a temperature of the hydrogen gas atan upstream of the hydrogen pressure regulator valve.
 12. The fuel cellsystem according to claim 7, wherein the consumption rate of thehydrogen gas except for the exhaust hydrogen gas discharged from thepurge valve is obtained as a value that corresponds to a flow rate ofthe hydrogen gas to be used for the fuel cell to generate electricpower.
 13. The fuel cell system according to claim 12, wherein theconsumption rate of the hydrogen gas except for the exhaust hydrogen gasdischarged from the purge valve is obtained based on output current ofthe fuel cell.
 14. The fuel cell system according to claim 12, whereinthe consumption rate of the hydrogen gas except for the exhaust hydrogengas discharged from the purge valve is obtained based on at least one of(i) a pressure of the hydrogen gas at an inlet of the fuel cell and (ii)a pressure of the hydrogen gas at an outlet of the fuel cell.
 15. A fuelcell system comprising: a fuel cell supplied with hydrogen gas andoxidizing gas; gas supply means supplying the hydrogen gas to the fuelcell; gas discharging means for discharging exhaust hydrogen gasexpelled from the fuel cell to outside, the exhaust hydrogen gascontaining nitrogen in addition to hydrogen and the gas dischargingmeans including purge means for purging the exhaust hydrogen gas,containing nitrogen, to the outside; gas recirculation means forrecirculating the exhaust hydrogen gas to the fuel cell at an upstreamthereof; and control means configured to calculate a flow rate of theexhaust hydrogen gas passing through the purge means and controlling thepurge means so as to close the purge means when the control means judgesthat the flow rate of the exhaust hydrogen gas calculated thereby isequal to or greater than a predetermined value.
 16. A method ofcontrolling a fuel cell system, which has a fuel cell supplied withhydrogen gas and oxidizing gas, a gas supply line supplying the hydrogengas to the fuel cell, a gas exhaust line passing exhaust hydrogen gasexpelled from the fuel cell with the exhaust hydrogen gas containingnitrogen in addition to hydrogen, a gas recirculation line recirculatingthe exhaust hydrogen gas to the fuel cell at an upstream thereof and apurge valve adapted to discharge the exhaust hydrogen gas, containingnitrogen, to outside, the method comprising: calculating a flow rate ofthe exhaust hydrogen gas passing through the purge valve; and closingthe purge valve when the calculated flow rate of the exhaust hydrogengas is equal to or greater than a predetermined value.
 17. The method ofclaim 16, further comprising using a pressure sensor to detect apressure of the hydrogen gas in the gas supply line, and setting thepredetermined value for closing the purge valve to decrease as thepressure of the hydrogen gas detected by the pressure sensor increases.18. The method of claim 16, further comprising obtaining the flow rateof the exhaust hydrogen gas passing through the purge valve by taking adifference between a supply rate of the hydrogen gas, supplied to thefuel cell, and a consumption rate of the hydrogen gas, among thehydrogen gas consumed in the fuel cell, except for the exhaust hydrogengas discharged from the purge valve.
 19. The method of claim 18, furthercomprising obtaining the consumption rate of the hydrogen gas except forthe exhaust hydrogen gas discharged from the purge valve as a value thatcorresponds to a flow rate of the hydrogen gas to be used for the fuelcell to generate electric power.
 20. The method of claim 19, furthercomprising obtaining the consumption rate of the hydrogen gas except forthe exhaust hydrogen gas discharged from the purge valve based on outputcurrent of the fuel cell.
 21. The method of claim 19, further comprisingobtaining the consumption rate of the hydrogen gas except for theexhaust hydrogen gas discharged from the purge valve based on at leastone of (i) a pressure of the hydrogen gas at an inlet of the fuel celland (ii) a pressure of the hydrogen gas at an outlet of the fuel cell.